Most athletes are admired for their strength and stamina. But in his latestbook, Why Michael Couldn't Hit (W. H. Freeman, $22.95), neurologist Harold Klawans, M.D., says that success on the playing field often depends as much on brain as on brawn. A little neuroscience, it seems, explains some of the woes even the greatest athletes have endured. In other instances, neurological quirks may have contributed to sports stardom. Some examples:

WINNERS

o Mahmoud Abdul-Rauf. The Denver Nuggets guard, one of basketball's best free throw shooters, suffers from Tourette's syndrome. Coincidence? Maybe not, says Dr. Klawans. Although Tourette's is best known for causing involuntary swearing, a more common symptom is ritualistic, compulsive behavior. As a teen Abdul-Rauf felt compelled to shoot free throws for hours until he hit 10 straight. And not just any 10 baskets--10 that sounded right. All that compulsive practice is now paying off.

o Wayne Gretzky. Over his career, Gretzky has racked up more scoring records than many players have goals. One reason: his turbo-charged neurons. Gretzky's "long loop" reflexes--muscle movements triggered by lengthy loops of brain cells--are the fastest ever measured by a Canadian expert. So the Great One is able to get off a shot more quickly than the rest of us mere mortals.

LOSERS

o Michael Jordan. Why did the basketball superstar strike out when he switched to baseball a few years back? Because there's often a limited window of time during which the brain can master a skill. Just as learning a second language is easiest when we're young, so it goes with whacking a sphere. While Jordan certainly played baseball better than most humans, to achieve major-league success he should have started batting practice at least 10 years earlier.

o Muhammad Ali. Sadly, the former champ has developed the muscular stiffness and tremors of Parkinson's disease. The reason, Dr. Klawans suggests, is that the ex-heavyweight was born with a featherweight substantia nigra--a brain area that controls movement. Most of us can lose half of this region without ill effects. But Ali's already-stunted nigra left him vulnerable to pugilistic pounding.